1. Field of the Invention
The invention relates to pressure swing adsorption processing. More particularly, it relates to enhanced pressure swing adsorption processes for the separation of air.
2. Description of the Prior Art
In numerous chemical processing, refinery, metal production and other industrial operations, high purity oxygen and nitrogen streams are used for a variety of purposes. Thus, high purity nitrogen is used for purging, blanketing, the providing of metal treating atmospheres and other purposes. High purity oxygen is used in chemical processing and steel and paper mill applications, lead and glass production operations and the like. Nitrogen and oxygen are produced from air, typically by cryogenic distillation processing. While such processes can be very efficient, particularly for large gas volume applications, they require the use of complex and costly cryogenic processing equipment. Pressure swing adsorption (PSA) processing is particularly suited for such air separation operations, particularly for relatively small gas volume applications where a cryogenic air separation plant may not be economically feasible.
In the PSA process, as used for air separation, feed air is commonly passed to an adsorbent bed capable of selectively adsorbing nitrogen as the more readily adsorbable component of air at an upper adsorption pressure. Oxygen, as the less readily adsorbed component of air, is passed through and discharged from the bed. The bed is thereafter depressurized to a lower desorption pressure for desorption of said nitrogen and its removal from the bed prior to the introduction of additional quantities of feed air to the bed as cyclic adsorption-desorption operations are continued in said bed. As those skilled in the art will readily appreciate, the PSA process is commonly employed in multi-bed systems, with each bed undergoing the desired PSA processing sequence on a cyclic basis interrelated to the carrying out of such processing sequence in the other beds of the system.
A typical PSA air separation processing cycle consists of four processing steps; namely, (1) passage of feed air to the teed end of the bed, at the upper adsorption pressure, for selective adsorption of nitrogen and passage of oxygen from the opposite or discharge end or the bed; (2) depressurization or "blowdown" of the bed to the lower desorption pressure, with desorption of nitrogen and its removal from the feed end of the bed; (3) purge by the introduction of a purge stream to the bed from the discharge end thereof to further desorb and remove nitrogen from the feed end of the bed; and (4) repressurization of the bed to the upper adsorption pressure, with such sequence being repeated on a cyclic basis as additional quantities of feed air are passed to the PSA system on a continuous basis. Various modifications are known with respect to such PSA processing, with additional processing steps, such as full or partial pressure equalization steps for pressure recovery, being included, and other steps, such as the purge step, being omitted in particular PSA processing cycles.
In PSA processes based on the selective adsorption of nitrogen from air, commercially available adsorbent materials capable of selectively adsorbing nitrogen from air are employed in the adsorbent beds of the PSA system. Well known molecular sieves, such as 13X, 5A, and 10X and mordenite, are representative examples of adsorbent materials that can conveniently be employed for such PSA air separation processing. Molecular sieve materials generally tend to exhibit a large internal surface and thus have a high capacity for the selective adsorption of nitrogen from air. They are also polar in nature, which property leads to the selective adsorption of nitrogen relative to oxygen. Such molecular sieve materials are complex "framework" structures that can exist in many different structural modifications. In addition, the polar ions inherent in zeolitic molecular sieves can be modified by ion-exchange processing. Thus, there are many different molecular sieve adsorbent materials known in the art that are more or less satisfactory for use in PSA air separation processes. The selective or preferential adsorption of nitrogen, relative to oxygen, may be quantified in terms of "separation factor". There have been efforts in the art to develop particular adsorbent materials having a high separation factor for air separation operations, together with a high adsorptive capacity for the selectively adsorbed nitrogen.
The sodium form of the faujasite-type Zeolite X has often been used to advantage in PSA air separation processes. It has been suggested that improved adsorption of nitrogen can be achieved by exchanging the sodium ions with divalent ions. McKee, U.S. Pat. No. 3,140,932, discloses high separation factors for nitrogen to oxygen for CaX, SrX, BaX and NiX. Sircar et al, U.S. Pat. No. 4,557,736, disclose that a binary ion exchanged X-zeolite, with both Ca and Sr ions, has a particularly high adsorption of nitrogen at superatmospheric pressure, with 5-40% of the cations being Ca.sup.++ and 60-95% being Sr.sup.++ in preferred embodiments.
The McKee patent referred to above considers the relative merits of various alkali metal cation forms of zeolite X, and indicates that the Li.sup.+ form is superior for the selective adsorption of nitrogen from air. This form of adsorbent was found to have a high adsorptive capacity, even at temperatures as high as 0.degree. C. Nitrogen to oxygen separation factors as high as 6.8 were measured. Furthermore, the loading separation factor actually increased with increasing temperature. More recently, Chao, U.S. Pat. No. 4,559,217, disclosed that highly lithium-exchanged zeolite X, with at least 88% of the cation sites occupied by Li.sup.+, has a higher than expected separation factor for nitrogen from air in conventional PSA processing operations, with separation factors as high as 10.9 at one atmosphere adsorption pressure and ambient pressure conditions. Furthermore, high differential nitrogen loadings on the adsorbent material were found for adsorption at 1500 torr as compared to 150 torr.
It is known in the art, therefore, that various modifications can be made to the zeolite structure of desirable adsorbent materials to enhance the selective adsorption of nitrogen from air. By the use of such modified adsorbents, it is thus Possible to improve the adsorption step of PSA air separation processes. While this represents a desirable advance in the art, it does not, unfortunately, necessarily result in an improvement in the overall PSA air separation processing. Adsorbents that tend to more effectively and selectively adsorb nitrogen from air under upper adsorption pressure conditions also tend to hold the nitrogen more strongly under the lower pressure desorption conditions. Moreover, the overall cost and efficiency of the PSA process may depend as much on the desorption steps as on the adsorption steps of the overall process. This is equally true when the desorption operation is carried out under vacuum conditions or at about atmospheric conditions.
If efficient utilization of the adsorbent were the only pertinent consideration, it would be desirable to completely desorb the selectively adsorbed nitrogen during the bed regeneration portion of each PSA cycle, so that the total loading capacity of the adsorbent would be available during the next succeeding adsorption portion of the cycle. The best adsorbent for use in such a process would be a material that exhibited the highest nitrogen loading capacity and the largest nitrogen-to-oxygen separation conditions under the desired adsorption conditions. Complete desorption would require a very deep vacuum. The equipment needed to achieve such deep vacuum conditions is costly, and the operating costs associated with such operations are very high. Practical PSA processing operations, particularly those designed for desirable power efficiency, must operate with a partial desorption operation carried out at a lower desorption pressure level that is, nevertheless, well above the deep vacuum conditions that would be needed for complete desorption.
In meeting the ever more stringent requirements of industrial activities, PSA air separation operations are dependent on the continued development of advanced adsorbent materials for the selective adsorption of nitrogen under practical operating conditions. In turn, the effective use of such advanced adsorbent materials requires the development of PSA operating features so as to achieve overall PSA air separation performance capable of fulfilling the requirements of practical commercial operations. As indicated above, a variety of PSA processing cycles and features are known in the art. The overall efficiency of each PSA system and processing cycle will be understood to depend upon the particular features thereof. However, the dominant factor respecting the total energy requirements of a particular PSA operation is the pressure ratio of the maximum upper adsorption pressure to the minimum lower desorption pressure. Various developments have been made to advance the PSA air separation art in the overall direction of lower cost, more efficient separation operations. Thus, Lagree and Leavitt, U.S. Pat. No. 4,810,265, disclose a two-bed vacuum PSA process and system for the production of nitrogen from air, utilizing a cocurrent product purge and a countercurrent oxygen purge, developed for low power consumption and capital costs. Improved vacuum PSA processing for the production of oxygen from air has also been proposed, utilizing simple processing equipment, having low capital costs, which can be operated at power consumption levels similar to, or lower than, other commercial PSA processes.
Despite such considerable advances in the PSA art for the separation of air into oxygen and/or nitrogen product streams, there is a genuine need in the art for more efficient PSA air separation processes. Such need, to satisfy the demands of existing and contemplated industrial applications for such oxygen and/or nitrogen products, necessitates the development of more efficient processes for air separation to produce high purity oxygen and nitrogen products, particularly at very low power consumption levels.
It is an object of the invention, therefore, to provide an improved PSA process for the air separation applications.
It is another object of the invention to provide an improved PSA process for the production of oxygen (and argon) and nitrogen component streams at advantageously low power consumption levels.
It is a further object of the invention to provide an improved process for the production of high purity oxygen.
It is a further object of the invention to provide a PSA process for air separation capable of achieving very low power consumption, together with capital costs similar to, or lower than, those associated with conventional PSA operations.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.